In many applications, bearings encounter hostile, corrosive operating environments and high repetition use. Corrosion, rolling contact fatigue, and loss of lubricant may reduce bearing life. Corrosion of steel in bearings may result in high maintenance costs and equipment down time while costly repairs or replacements are made. The high repetition inherent in bearing use causes fatigue in the bearing elements and the raceways. Additionally, in use, lubricant escapes from bearings, resulting in frequent maintenance and possible equipment downtime while lubricant is reapplied. It is thus desirable to have an invention that protects against corrosion, promotes long fatigue life, and can be applied to machine elements with minimum equipment downtime.
One known method of corrosion protection is to cover a material with a protective coating to physically separate the material from exposure to a potentially corrosive environment. For example, methods have been taught to combat corrosion through the application of a zinc plating to raceway and roller bearing surfaces. As disclosed in U.S. Pat. No. 5,352,046, for example, zinc or zinc alloy plated layers are applied to contact surfaces by an electroplating process. This electroplating process is disadvantageous, however, because it adds expense and time to the manufacturing process. Corrosion protection of bearings and raceways has also been attempted through multilayer plating and protection systems, as disclosed in U.S. Pat. No. 6,062,735, for example. Disadvantageously, the application of these multiple layers requires even more time and manufacturing expense than the application of a single plated layer of zinc or zinc alloy. In addition, neither U.S. Pat. No. 5,352,046 nor U.S. Pat. No. 6,062,735 discloses protection that can be readily applied or retrofitted to bearings already in use.
Another method of corrosion protection is anodic protection. Anodic protection involves the use of a more anodic, or active, metal to coat and protect a metal that is less anodic. In some applications, one advantage of anodic protection is that breaks or scratches in the more anodic metal coating do not cause the underlying less anodic metal to corrode.
The term anodic metal refers to the tendency of dissimilar materials to ionize or corrode. The relative corrosive tendencies of many dissimilar metals and metal alloys are predictable and have been tabulated in what is commonly known as a galvanic series, an example of which can be found at ASTM G82-98.
Metals and metal alloys are arranged in the galvanic series from least anodic to most anodic. When dissimilar metals or metal alloys are coupled such that electrons can flow between them, electrons will flow from the more anodic metal or metal alloy to the less anodic metal or metal alloy. This electron flow is accompanied by ionization of the more anodic metal or metal alloy that leads to corrosion of the more anodic metal or metal alloy. Consequently, the more anodic metal or metal alloy can corrode but the underlying, less anodic material does not.
Other inventions have sought to increase lubricant retention through the use of solid lubricants or metal-containing pastes. However, as disclosed in U.S. Pat. No. 4,363,737, for example, these pastes do not provide anodic protection against corrosion. This patent also ignores the necessity to entrain soft solid metal particles in a lubricated contact in order to form transfer films on the contacting bodies. The patent recommends a metal particle content of at least thirty-five percent by weight, and does not specify the size of the metal particles in the paste. Both particle size and concentration strongly affect entrainment of particles. In addition, Qiu, “Preparation of Ni Nanoparticles and Evaluation of Their Tribological Performance As Potential Additives in Oils,” discloses the use of Nickel nanoparticles as additives in lubricating oils to improve their load bearing capacity and friction and wear characteristics. Qiu does not, however, disclose the use of nanoparticles to provide anodic protection against corrosion.
Accordingly, there is a need for a lubricant for bearings and other machine elements that provides anodic protection against corrosion. In addition, there is a need to provide a lubricant for bearings and other machine parts that provides a soft coating on bearing raceways to improve rolling contact fatigue life. There is also a need to provide a lubricant for bearings and other machine elements that is easy to apply to bearings that are already in use.